Field
Various communication systems may benefit from codebook methods and devices for multiple transmitters. For example, a codebook for four transmitters (4Tx) may provide further enhancement for downlink multiple-input multiple-output (DL-MIMO) systems.
Description of the Related Art
Downlink multiple-input multiple-output (DL-MIMO) can be supported in a variety of ways, for example with the transparent method described in U.S. patent application Ser. No. 14/158,035, filed Jan. 17, 2014, the entirety of which is hereby incorporated herein by reference. The transparent method for four transmitters (4Tx) may rely on an eight transmitter (8Tx) channel state information (CSI) feedback scheme.
However, 8TX codebook support as such may be limited to Transmission Mode (TM) 9/10. Also, 8TX codebook (CB) support from a user equipment (UE) point of view is currently an optional feature.
In Rel-10 8Tx codebook design, the beam granularity for ranks 3 and 4 is different from that for ranks 1 and 2 and the beam granularity for rank 1 is the same as that for rank 2. In Rel-12, it may be that precoders will be included that are matched to large angle spread and small angle spread in the codebook so the codebook's performance is robust for different propagation scenarios.
The long term evolution (LTE) Rel-10 8Tx codebook is a so-called “dual codebook” in the sense that each codeword is defined as a product of two matrices, the first matrix comes from codebook one (C1), and the second matrix comes from codebook two (C2). Refer to 3GPP R1-104473, “Way forward on 8Tx Codebook for Rel.10 DL MIMO”, 23-27 Aug. 2010, Madrid, Spain, which is hereby incorporated herein by reference in its entirety.
C1 can be defined as follows. First, a 4×32 matrix B can be defined as
B=[b0 b1 . . . b31],
and elements of B can be defined as
                    [        B        ]                              1          +          m                ,                  1          +          n                      =          ⅇ              j        ⁢                                  ⁢                              2            ⁢            π            ⁢                                                  ⁢            mn                    32                      ,m=0, 1, 2, 3, n=0, 1, . . . , 31.
X(k)ε{[b2k mod 32 b(2k+1)mod 32 b(2k+2)mod 32 b(2k+3)mod 32], k=0, 1, . . . , 15}; and X(k) can be a 4×4 matrix.
Moreover, W1 can be defined as
            W      1              (        k        )              =                  [                                                            X                                  (                  k                  )                                                                                                                                                                                                                                                X                                  (                  k                  )                                                                    ]                    8        ×        8              ,such that codebook 1 is defined as C1={W1(0), W1(1), W1(2), . . . , W1(15)}.
For rank 1 and rank 2, C2 can be defined differently. Thus, for rank 1:
                    W        2            ∈              C        2              =          {                                    1                          2                                ⁡                      [                                                            Y                                                                              Y                                                      ]                          ,                              1                          2                                ⁡                      [                                                            Y                                                                                                  j                    ⁢                                                                                  ⁢                    Y                                                                        ]                          ,                              1                          2                                ⁡                      [                                                            Y                                                                                                  -                    Y                                                                        ]                          ,                              1                                          2                            ⁢                                                                            ⁡                      [                                                            Y                                                                                                                        -                      j                                        ⁢                                                                                  ⁢                    Y                                                                        ]                              }        ,          ⁢      Y    ∈                  {                                            e              ~                        1                    ,                                    e              ~                        2                    ,                                    e              ~                        3                    ,                                    e              ~                        4                          }            .      
However, for rank 2:
                    ⁢                            W          2                ∈                  C          2                    =              {                                            1                              2                                      ⁡                          [                                                                                          Y                      1                                                                                                  Y                      2                                                                                                                                  Y                      1                                                                                                  -                                              Y                        2                                                                                                        ]                                ,                                    1                              2                                      ⁡                          [                                                                                          Y                      1                                                                                                  Y                      2                                                                                                                                  j                      ⁢                                                                                          ⁢                                              Y                        1                                                                                                                                                -                        j                                            ⁢                                                                                          ⁢                                              Y                        2                                                                                                        ]                                      }                                ⁢                  and        ⁢                                  (                              Y            1                    ,                      Y            2                          )            ∈              {                              (                                                            e                  ~                                1                            ,                                                e                  ~                                1                                      )                    ,                      (                                                            e                  ~                                2                            ,                                                e                  ~                                2                                      )                    ,                      (                                                            e                  ~                                3                            ,                                                e                  ~                                3                                      )                    ,                      (                                                            e                  ~                                4                            ,                                                e                  ~                                4                                      )                    ,                      (                                                            e                  ~                                1                            ,                                                e                  ~                                2                                      )                    ,                      (                                                            e                  ~                                2                            ,                                                e                  ~                                3                                      )                    ,                      (                                                            e                  ~                                1                            ,                                                e                  ~                                4                                      )                    ,                      (                                                            e                  ~                                2                            ,                                                e                  ~                                4                                      )                          }                                ⁢    where                      ⁢                                        e            ~                    1                =                  [                                                    1                                                                    0                                                                    0                                                                    0                                              ]                    ,                                    e            ~                    2                =                  [                                                    0                                                                    1                                                                    0                                                                    0                                              ]                    ,                                    e            ~                    3                =                  [                                                    0                                                                    0                                                                    1                                                                    0                                              ]                    ,                                    e            ~                    4                =                              [                                                            0                                                                              0                                                                              0                                                                              1                                                      ]                    .                    
Various assumptions may be used in codebook design for communication systems. For example, a new aperiodic physical uplink shared channel (PUSCH) feedback mode may be supported. The feedback can include channel quality indicator (CQI) and rank feedback bit size as in PUSCH Mode 3-1 in release (Rel) 10 of the third generation partnership project (3GPP). The feedback can also include a wideband precoding matrix indicator (PMI), which includes, for two transmitters (2Tx): 0 bit, for 4Tx: various possibilities, for 8TX: 4/4/2/2/2/2/2/0 bits for rank 1-8 respectively. The feedback may also include per subband PMI(s), for 2Tx: 2/1 bits for rank 1-2, 4Tx: various possibilities, for 8Tx: 4/4/4/3/0/0/0/0 bits for rank 1-8 respectively.
It may also be assumed that a Rel 10 dual codebook structure (W=W1W2) can be used in new codebook in Release 12 for 4 antenna feedback for demodulation reference signal (DMRS) based TMs. Other features may vary, such as subband size and the detailed W1 and W2 structures. For example, W1 can correspond to a long term and/or wideband channel properties and W2 can correspond to a short-term and narrowband channel. Likewise, there may be additional information in the CSI reports for this new feedback mode. For example, there may be CSI feedback enhancements targeted at improving multi-user (MU) performance. Furthermore, Rel-10 4tx codebook can be also expressed with the dual codebook structure (W=W1W2) with W1 being the identity matrix.
The following codebook structure is defined for Rel-10. For all ranks 1 to 8, W1=[X 0;0 X], which is block diagonal, W=W1*W2, with block diagonal W1 matching the spatial covariance of dual-polarized antenna setup with any spacing (e.g. 1/2 wavelength or 4 wavelength), with at least sixteen 8Tx discrete Fourier transform (DFT) vectors generated from W1 and co-phasing via W2 matching the spatial covariance of ULA antenna setup, and with good performance for high and low spatial correlation. For rank 1 to 4: X is 4xNb matrix. Moreover, there can be 32 4Tx DFT beams for X. Furthermore, the beam index can be 0, 1, 2, . . . , 31. Furthermore, for each W1, adjacent overlapping beams can be used to reduce edge effect in frequency-selective precoding, and thus ensure the same W1 is “optimal” for sub-bands with potentially different W2.
For rank 1 and 2, W1 Nb=4 adjacent overlapping beams with eight W1 matrices per rank: {0,1,2,3}, {2,3,4,5}, {4,5,6,7}, . . . , {28,29,30,31}, {30,31,0,1}. Moreover, beam selection and co-phasing can be supported by W2, which can provide 16 combinations at rank 1 and rank 2 respectively (at rank 1, 4 beam selection choices and 4 QPSK co-phasing choices lead to 16 combinations, and at rank 2, 8 beam selection choices and 2 QPSK co-phasing choices lead to 16 combinations).
In 3GPP TS 36.213 (Rel-10 and Rel-11) and TS 36.212 (Rel-10 and Rel-11), to use the dual codebook in the periodic feedback mode 1-1, two submodes (submode 1 and submode 2) with codebook subsampling are introduced. Also, to support the dual codebook in the periodic mode 2-1, codebook subsampling is used for rank 2, rank 3 and rank 4 (for rank 1, there are enough bits for both W1 and W2, subsampling is not used). “Subsampling” here means in a codebook only some combinations of W1 and W2 are eligible to be selected by a user equipment in a feedback mode so there are fewer bits needed to represent W1 and W2.
As the beam group definition is identical for rank 1 and rank 2 in the Rel-10 8Tx codebook, the subsampling is identical for both in submode 1 and submode 2 of periodic feedback mode 1-1.